Localization to the appropriate subcellular compartment is of critical importance for the functioning of many different sorts of proteins. The eukaryotic cell typically employs several different strategies for the targeting of proteins to sites where their functions are required. One of these is mRNA localization. In this targeting strategy, specific mRNAs are first transported to particular intracellular compartments. The mRNAs are anchored at these target sites where they can be stored in a "masked" form until they are translated into protein in response to some type of signaling pathway. This mRNA localization strategy not only permits the production of compartmentalized proteins "on site", but is often also coupled to mechanisms which can coordinate the timing and/or level of synthesis of these proteins with other biological processes that are occurring inside the cell. mRNA localization appears to be a particularly important strategy for protein targeting in developmental systems where positional information (specifying different cell fates) must be distributed in a defined spatial array and where proteins must be synthesized in temporal patterns that are coordinated with other developomental events. The experiments described in this proposal focus on the orb gene. This gene plays a key role in several different mRNA localization pathways during Drosophila oogenesis, controlling the "on site" translation of localized mRNAs that are required to establish polarity axes in the Drosophila egg and embryo.